Method for enhancing the performance of steel reheat furnaces

ABSTRACT

Methods for treating a steel billets with carbon black in order to increase quality. This increase in emissivity will enhance the radiative heat transfer to the billet, and also improve the temperature homogeneity of the billet, thus improving the production rate and quality. The carbon black layer on the surface will act as a barrier to prevent or slow the rate of scale formation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 60/635,259, filed Dec. 10, 2004, the entire contents of which are incorporated herein by reference.

BACKGROUND

A typical manufacturer of steel products will cast molten steel into intermediate products such as slabs, blooms or billets. These intermediate products are rolled to shape in a steel mill. In order to facilitate the rolling process, the steel is heated to a temperature in the range of 1000° C. to 1325° C. in a reheat furnace.

Steel manufactures using reheat furnaces to achieve a homogenous billet temperature before processing on the forming line are concerned with scale formation, decarburization, production rate, and NOx pollutants. Scale formation results from the billet exposure to high temperatures (T>1000° C.) with oxidizers present. The level of scale formation can range between 1%-2% and represents a product loss that can be costly. For example, a 1 million ton operation with a 1% yield loss represents a $4MM loss. In addition, excessive scaling in the reheat furnace leads to faster scale buildup in the furnace hearth requiring higher maintenance frequency for cleaning. For decarburization, the outer surface layer exhibits a reduction in carbon concentration that can impact the steel quality in the forming operation. This process occurs by the diffusion of carbon to the interface and is dependent on the scale and furnace atmosphere. For the production rate, the reheat furnace is typically identified as bottleneck limited by the design power of the furnace, which sets the throughput rate of steel. Finally, environmental regulations for the level of NOx restrict the production rate, require trading of NOx credits, and requires capital cost for pollutant abatement equipment.

To address these issues operators have limited options that mainly focus around controlling the temperature profile and gas atmosphere in the furnace. For example, in controlling scale formation reheat furnaces operate with excess O₂ between 2%-4% and in some cases operate under reducing atmospheres. Productivity increases are handled by altering the burner power distribution or adding additional power in different zones. Additional control of the process is obtained using process control to regulate the different furnace zones at optimum conditions. The operation of a reheat furnace can be considered continuous or semi-batch since the time profile, i.e., residence time, of the product entering the furnace can vary. Improper operation of the furnace can result in scaling and decarburization problems. Both of these phenomena are functions of temperature and time and overheating and long duration of the load in the furnace should be avoided.

SUMMARY

This invention relates to a method of coating steel billets, either prior to the introduction to a reheat furnace, or in-situ during the reheat process. More particularly, the present invention relates to a method for achieving homogeneous billet temperature and to reduce the resulting scale formation on the billet.

The present invention is directed to overcoming, or at least reducing, the effects of one or more of the problems set forth above.

One aspect of the present invention provides a method of treating a steel billet. The treatment method includes providing a source of highly-carbon-laden gas; providing a steel workpiece; directing said highly-carbon-laden gas to at least one surface of said steel workpiece, thereby producing a coated steel workpiece; and introducing said coated steel workpiece to a steelmaking process.

A second aspect of the present invention provides a method of treating a steel billet. The treatment method includes providing a source of a highly-carbon-laden gas, providing a steel workpiece; introducing said steel workpiece to a first steelmaking process, directing said highly-carbon-laden gas to at least one surface of said steel workpiece, thereby producing a coated steel workpiece, and introducing said steel workpiece to a second steelmaking process.

A third aspect of the present invention provides a method of treating a steel billet. The treatment method includes providing a source of a highly-carbon-laden gas; providing a steel workpiece, introducing said steel workpiece to a heating zone in a reheat furnace, directing said highly-carbon-laden gas to at least one surface of said steel workpiece, thereby producing a coated steel workpiece, and introducing said steel workpiece to a soak zone in said reheat furnace.

A fourth aspect of the present invention provides a method of treating a steel billet. The treatment method includes providing a steel workpiece, introducing said steel workpiece to a heating zone in a reheat furnace, providing a source of a gas mixture, reacting said gas mixture within said heating zone to produce a highly-carbon-laden gas, directing said highly-carbon-laden gas to at least one surface of said steel workpiece, thereby producing a coated steel workpiece, and introducing said steel workpiece to a second steelmaking process. The gas mixture may be selected from the group consisting of acetylene and a mixture of acetylene and an oxidant.

This highly-carbon-laden gas may be carbon black. This steelmaking process may be the introduction into a reheat furnace. This coated steel workpiece may have an emissivity of greater than about 0.8. This coated steel workpiece may have an emissivity of greater than about 0.9. This may have an emissivity of between about 0.8 and about 0.95.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects of the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

FIG. 1 is a schematic billet treatment method in accordance with one illustrative embodiment of the present invention;

FIG. 2 is a schematic billet treatment method in accordance with another illustrative embodiment; and

FIG. 3 is a schematic billet treatment method in accordance with yet another illustrative embodiment.

DESCRIPTION OF PREFERRED EMBODIMENTS

As used in this application, the term “billet” is defined to mean an ingot, slab, bloom, billet, or any form of semi-finished metal that are subsequently processed in a steelmaking finishing facility. As used in this application, the term “finishing facility” is defined to mean the portion of a steelmaking complex that processes semi-finished steel to forms that can be used by others, which can include, but is not limited to, rolling mills, pickle lines, tandem mills, annealing facilities and temper mills.

The combustion of fuel in the reheat furnace provides the thermal energy to the steel load to obtain the desired temperature homogeneity before the product enters the forming line. Under complete combustion, the gaseous products CO₂ and H₂O are in contact with the steel load. The temperature and duration of the atmosphere impacts the degree of scaling that occurs on the metal surface.

The CO₂ in the atmosphere will react with the carbon in the steel to produce CO at the austenitizing temperatures (T>900° C.) by the following reaction: C+CO₂=→2CO

This reaction continues until there is a complete decarburization of the steel surface. In the presence of oxygen, e.g., from air, the oxidation of Fe occurs as follows: 2Fe+O₂=2FeO 3Fe+2O₂=Fe₃O₄ 4Fe+3O₂=2Fe₂O₃

Furthermore, the mixture of CO and CO₂ in the atmosphere contributes to scale by the reversible reaction: Fe+CO₂⇄FeO+CO

The presence of H₂O vapor in the furnace will also react with Fe to contribute to scale formation through the reaction: Fe+H₂O⇄FeO+H₂

In the decarburization process the formation of CO and CO₂ in the carbon steel effects the rate of iron diffusion out of the base metal, which is more favorable for oxidation to Fe₂O₄ and Fe₂O₃. Competing effects of decarburization and scale formation results in the concentration of the metal to decrease within the scale layer from inside to outside. The growth of the scale layer is dependent on the diffusion rates through the layers, gradient of the chemical potentials across the layers, and the relative porosity of the layers.

The complexity of the scale formation and decarburization processes has been handled mainly through improved furnace control. This is achieved by monitoring and controlling the furnace atmosphere conditions, and/or controlling the temperature and residence time of the steel load. With this limited range of control options, puts a constraint on the minimum achievable scale and/or decarburization.

With the present invention, the problems faced by steel manufactures are addressed by applying a carbon black film to the billet. The carbon film provides two main benefits to the process.

First, the emissivity values for steel range from about 0.5 to about 0.8 depending on the level of oxidation and roughness. An increase in the emissivity of the steel up to about 0.95 can be achieved by adding the carbon deposit to the surface. This change in emissivity will enhance the radiative heat transfer to the billet along with improving the temperature homogeneity of the billet, thus improving the production rate and quality.

Second, the carbon layer on the surface will act as a barrier to prevent or slow the rate of scale formation. The rate of removal of the carbon layer will depend on the O₂ concentration, temperature, and residence time of the billet. The lifetime of the carbon layer can be regulated by controlling the different zones of the furnace to achieve the desired atmosphere to minimize the oxidation. Similarly, removal of the carbon layer can be obtained by increasing the oxidation rate of the carbon by controlling the oxygen level in the furnace atmosphere. Residual carbon on the billet that leaves the reheat furnace will be removed in the descaling process.

Other potential benefits are pollutant control through the generation of CO by oxidation of the carbon layer resulting in NOx reduction. Finally, a reduction in decarburization can be achieved due to the high carbon concentration at the metal carbon film interface.

Turning to FIG. 1, in one embodiment, the billet may be surrounded by several burners. This arrangement may allow all sides of the billet to be coated with carbon black. These burners may be comprised of an array of jets that carry O₂, NG, and C₂H₂. As shown in FIG. 2, the O₂ and NG jets may produce a pilot flame that is used to combust the C₂H₂ resulting in a fuel rich flame producing carbon. In another embodiment, the combustion of the C₂H₂ may be initiated by the high temperature of the surrounding environment. The burners may be stationary while the billet is transported along a set of rollers. Either a single set of burners or multiple sets of burners may be used. Multiple burners would effectively increase the thickness of the carbon coating, thereby improving the protective barrier. In addition, the thicker coating will increase the lifetime of the protective barrier to further reduce the oxidation or decarburization of the surface.

The emissivity of the treated billet may be greater than about 0.8. The emissivity may be between about 0.8 and about 0.95. The emissivity may be greater than about 0.9. Applying the carbon black coating to the billet can be made before the billet enters the reheat furnace or directly inside the reheat furnace. To apply the carbon coating on the billet before the entering the furnace a linear burner design can be used that surrounds the billet.

In another embodiment of the invention, the carbon coating may be applied directly in the furnace. FIG. 3 shows a schematic of a typical reheat furnace divide by the different zones. In this application, an array of burners are integrated directly into the furnace near the throat, which separates the heat and soak zones. This location has the advantage of being in close proximity to the billet. In addition, combustion of C₂H₂ may be initiated by the high temperature-surrounding environment.

Oxidation of the carbon coating will occur at temperatures above 600° C. with oxygen present. To minimize the loss in the carbon layer O₂ control in the atmosphere is desirable. This can be achieved by controlling the fuel/air ratio in the different zones of the process. Further optimization can be achieved by using in-situ atmosphere monitoring, e.g., tunable diode laser, near the billet surface to control the atmosphere.

Illustrative embodiments of the invention have been described above. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and have been herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer's specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 

1. A method of treating a steel billet comprising; a) providing a source of highly-carbon-laden gas; b) providing a steel workpiece; c) directing said highly-carbon-laden gas to at least one surface of said steel workpiece, thereby producing a coated steel workpiece; and d) introducing said coated steel workpiece to a steelmaking process.
 2. The method of claim 1, wherein said highly-carbon-laden gas is carbon black.
 3. The method of claim 1, wherein said steelmaking process is the introduction into a reheat furnace.
 4. The method of claim 1, wherein said coated steel workpiece has an emissivity of greater than about 0.8.
 5. The method of claim 1, wherein said coated steel workpiece has an emissivity of greater than about 0.9.
 6. The method of claim 1, wherein said coated steel workpiece has an emissivity of between about 0.8 and about 0.95.
 7. A method of treating a steel billet comprising; a) providing a source of a highly-carbon-laden gas; b) providing a steel workpiece; c) introducing said steel workpiece to a first steelmaking process; d) directing said highly-carbon-laden gas to at least one surface of said steel workpiece, thereby producing a coated steel workpiece; and e) introducing said steel workpiece to a second steelmaking process.
 8. The method of claim 7, wherein said highly-carbon-laden gas is carbon black.
 9. The method of claim 7, wherein said coated steel workpiece has an emissivity of greater than about 0.8.
 10. The method of claim 7, wherein said coated steel workpiece has an emissivity of greater than about 0.9.
 11. The method of claim 7, wherein said coated steel workpiece has an emissivity of between about 0.8 and about 0.95.
 12. A method of treating a steel billet comprising; a) providing a source of a highly-carbon-laden gas; b) providing a steel workpiece; c) introducing said steel workpiece to a heating zone in a reheat furnace; d) directing said highly-carbon-laden gas to at least one surface of said steel workpiece, thereby producing a coated steel workpiece; and e) introducing said steel workpiece to a soak zone in said reheat furnace.
 13. The method of claim 12, wherein said highly-carbon-laden gas is carbon black.
 14. The method of claim 12, wherein said coated steel workpiece has an emissivity of greater than about 0.8.
 15. The method of claim 12, wherein said coated steel workpiece has an emissivity of greater than about 0.9.
 16. The method of claim 12, wherein said coated steel workpiece has an emissivity of between about 0.8 and about 0.95.
 17. A method of treating a steel billet comprising; a) providing a steel workpiece; b) introducing said steel workpiece to a heating zone in a reheat furnace; c) providing a source of a gas mixture; d) reacting said gas mixture within said heating zone to produce a highly-carbon-laden gas; e) directing said highly-carbon-laden gas to at least one surface of said steel workpiece, thereby producing a coated steel workpiece; and f) introducing said steel workpiece to a second steelmaking process.
 18. The method of claim 17, wherein said gas mixture is selected from the group consisting of acetylene, a mixture of acetylene, and an oxidant.
 19. The method of claim 17, wherein said highly-carbon-laden gas is carbon black.
 20. The method of claim 17, wherein said coated steel workpiece has an emissivity of greater than about 0.8.
 21. The method of claim 17, wherein said coated steel workpiece has an emissivity of greater than about 0.9.
 22. The method of claim 17, wherein said coated steel workpiece has an emissivity of between about 0.8 and about 0.95. 